Aqueous alkali-metal alginate solution



Patented Mar. 7, 1950 UNITD STATES PATENT OFFICE AQUEOUS ALKALI-METAL ALGINATE SOLUTION British company No Drawing. Application May 5, 1948, Serial No. 25,316. In Great Britain June 24, 1947 Claims. 1

This invention relates to the production of alg-inate fibres, filaments, threads, bands and the like, hereinafter referred to generally as threads, by extruding an aqueous solution of an alkali alginate through a jet into a precipitating bath.

Alkali alginate solutions are usually produced for spinning purposes by dissolving the alginate in water to form a solution containing from about 5 toabout per cent of alginate. The viscosities of the solutions obtained vary according to the previous treatment which the alginate has received during the course of its isolation from seaweed and its purification and consequently it is not always possible to produce alginate solu-- tions having both the concentration and the viscosity desired for spinning purposes.

It frequently happens therefore that when a particular batch of alkali alginate is dissolved in water to form a Spinning solution, the viscosity of the solution obtained is so high that it is difficult to spin satisfactorily on a commercial scale.

Moreover, as the viscosity usually varies from.

batch to batch, spinning conditions which are suitable for one solution may require variation for the next spinning solution. This variation in viscosity also causes difiiculties when the solution is filtered since the higher the viscosity of the solution, the greater is the pressure required to force the solution through the filter press and the longer the time required for the filtration.

The viscosity of the alginate solutions is conveniently measured by the falling ball method. as the time, in seconds, taken for a steel ball of A; of an inch diameter to fall through centimetres of a column of the solution in a tube of 2 cm. diameter, the temperature of the solution being centigrade. It is usually desirable to spin alginate solutions having a ball fall within the range of about to about seconds whereas in practice a solution prepared by dissolving in water the required amount of commercial grade alkali alginate for thread manufacture is usually found to have a ball fall of a higher order and may be, for example, as high as 200 seconds. British specification No. 417,556 describes a method of producing alginic acid esters from seaweed in which 30 parts of the laminaria are extracted with a solution consisting of 15 parts of soda carbonate, 1 part of soda phosphate, 2 parts of soda sulphite, 17 parts of 20 per cent ammonia solution and 50! parts of water, and the extract after diluting with water is filtered to give a clear viscose solution. French specification No. 891,310

describes a process oi treating demineralised a1 gins with neutral salts of the alkali metals or of magnesium at a temperature of centigrade or a little higher to give a homogeneous pulp; it is stated that the alkali metal salt used may be a sulphite, and that it requires no more than dilu tion with a suflicient amount of water to obtain fluid solutions of alkali alginate. The solutions obtained by these prior proposals are, however, not suitable for spinning purposes and moreover the metal sulphites present do not act to reduce the viscosity of the solution.

The object of the present invention is to provide a process whereby alginate solutions of a viscosity suitable for spinning can be produced.

In accordance with the present invention, aqueous alkali alginate spinning solutions of a viscosity suitable for spinning are obtained by including a water-soluble salt of hyposulphurous acid (H2S2O4) in the solution. The salt may be added either to the solution of the alkali alginate in water or to the Water in which the alginate is to be dissolved. Suitable salts which may be used according to the invention are sodium hyposulphite otherwise known commercially as sodium hydrosulphite (Na2S2O4), potassium hyposulphite or ammonium hyposulphite but the sodium salt is preferred on account of its cheapness.

The present invention therefore provides a process for the production of aqueous alkali alginate spinning solutions which comprises forming a solution comprising water, an alkali algi' nate, and, as a viscosity reducing agent a watersoluble salt of hyposulphurous acid, preferably in an amount sufl'icient to reduce the ball fall of the solution, when determined by the method described herein, to within the range of about 35 to about '70 seconds.

In carrying out the process according to the invention, it is important that the alginate solution should not be so strongly acid as to cause decomposition of the hyposulphite, with the liberation of sulphur dioxide; for the usual industrial spinning process, the solution employed should have a pH value within the range of 6.0 to 9.5. Should the addition of the viscosity-reducing agent cause the pH to fall below this range, the pH may be raised by the addition of alkali.

It is believed that the lowering of the pH value of the solution occurs as a result of hydrolysis of the hyposulphite, resulting in the formation of acid sodium salts in the solution, and the alkali added in the manner described is believed to neutralise these acid salts, thus preventing an undue fall in the pH value.

The amount of alkali re- .quired to maintain the desiredalkalinity will gen:

erally be equal to or slightly less than that which is molecularly equivalent to the hyposulphite used.

The present invention therefore also includes a method of reducing the viscosity-of an aqueous alkali alglnate spinning solution containing from about to about per cent by weight of alginate which comprises incorporating in the solution, a water soluble salt of hyposulphurous acid in an amount sufficient to reduce the viscosity to the desired value whilst maintaining the pH value of the solution, if necessary by the addition of alkali, within the range of 6.0 to 9.5.

The method of reducing the viscosity of algi-- nate solutions according to the invention is particularl useful as a means 'for adjusting the viscosity of successive batches of alginate'spinning' solutions since by empirical tests .onsmall samples of the solutions, the required amount of hyposulphite necessary to reduce the viscosity of the solution to the preferred range of to 70 seconds can readily be determined. In this manner spinning conditions may be maintained constant over longperiods.

The invention therefore also includes a'meth'od of reducing the viscosity of an aqueous alkali alginate spinning solution containing from about 5 to about '15 per cent by Weight of alginate and having a ball-fall, when determined by the method described herein,liigher than 70 seconds,

which comprises incorporating in the solution, a water-soluble-salt of hyposulphurous acid in an amount sufficient to reduce the ball-fall of the solution to within the range of about 35 to about '70 seconds, whilst maintaining the pH valueof the solution, if necessary by the addition of alkali, within the rangeof 6.0 to 9.5.

The invention also includes an aqueous alkali alglnate spinning solution which contains from about 5 to about 15 per cent by weight of algihate and which also contains, as :a viscosityreducing agent, a water-soluble salt of hyposulphurous acid, the pH value of the said solution being within range of 6.0 to 9.5; the hyposulphite is preferably added in an amount suflicient to reduce the ball fall to within the range of 35 to .70 seconds.

The invention also includes .a process for the production of aiginate threads by extruding an aqueous solution of an alkali alginate through a jet into a precipitating bath, wherein the solu tion contains, as a viscosity-reducing agent, a water-s 'uble salt of hyposulphurous acid; the hyposulphite is preferably present in an amount suficient to reduce the ball fall to within the range of 35170 WO'seconds.

The amount of hyposulphite employed according to the invention is usually very small and norma ly does not exceed 1 per cent based on the weight of the solution. In some cases additions-oi as low as 0.02 percent of sodium hyposulphite will be sufficient to reduce the viscosity to the preferred spinning range of 3 to 70 seconds ball-fall.

The hyposulphite-may be incorporated directly in the alginate solution in order to bring about the desired viscosity reduction; alternatively, the desired amount of hyposulphite, as determined by a test on a small sample of the algihate solution, may be dissolved in Water and the alginate dissolved in the solution obtained so as to prepare an alginate solution of the desired viscosity directly; If it is necessar to raise the pH value by adding alkali, the alkali may be added to the hyposulphite before dis solving the alglnate or it may be added to the final alginate solution. In the first method of procedure, that is where the hyposulphite and alkali are first made into a solution and the alginate then dissolved inth'esolution obtained, it is advantageous to allow the alkaline aqueous hyposulphite solution to stand for a few minutes before adding the alginate.

In .thespinning of alginate threads, it is the usual practice to add a salt, usually sodium hexametaphosphate, to the spinning solution in order to maintain in solution any calcium ions which may be present as impurities in the alkali 'alginate. In carrying out the present invention, the sodium hexametaphosphate may be "added 'if desired to counteract any calcium ions present but it is not essential as the hyposulphite effects a reduction in viscosity in the presence or in the absence of sodium hexametaphosphate.

By reducing the viscosity of alginate spinning solutions in accordance with this invention, the time'taken for the pressure-filtering of the spinning solution may be considerably reduced and also :the forcing of the solution through the feed pipes ma be facilitated. In addition, the invention permits anappreciablesaving in power consumption for mixers and pumps. A further advantage of the present invention is that high viscosity types of alkali valginates may be rendered suitable for making alginate threads. Thus solutions which would normally have a ball-fall as high as 700 seconds and are thus unsuitable for spinning, may be treated in accordance with the invention to reduce the ballfall to below and so render the solution suitable for spinning.

The present invention and the order of reduction in viscosity possible are illustrated by the following examples, in which parts and percentagesare by weight:

Example 1 .A 7 per cent aqueous sodium alginate solution was prepared by dissolving a commercial sodium alginate in water and the solution (pl-I value 7.5) was divided into three parts. One part (A) was kept as a blank control and to the second part (B) and to the third part (C) 0.06 per cent and 0.15 percent respectively of sodium hyposulphite (NazSzoi), based on the weight of the solution, were added, the pH value of the solution being adjustecl'to 7.5 in each case by adding alkali in the form of a 25 per cent aqueous solution. Viscosity determinations car ried out in the manner described above gave ball-fall times of (A) 151 seconds, 13) 102 seconds and (C) 42 seconds. Solution (C) was readily spun into threads by the processes described in UnitedStates patent specification No. 2,423,075 and British specification No. 571.657.

Example 2 A 7,1Jer cent aqueous solution of sodium alginate from a different source from that of the sample used in Example 1 was found to have a ball-fall of 92 seconds. One sample of the solution was taken and .0.02,per cent, based on the weight ofv in spinningwas obtained.

Example 3 A further sample of commercial sodium alginate was dissolved in water to form a 7 per cent solution; this solution had a ball-fall of 94 seconds. A spinning solution using the same batch of sodium alginate was then prepared as follows:

100 parts of distilled water, 0.01 part of caustic soda and 0.035 part of sodium hyposulphite were mixed to form a solution, whereupon a quantity of sodium alginate sufiicient to form a '7 per cent solution was dissolved in the solution obtained. The alginate solution had a ball-fall of 35 seconds, the pH value being approximately 7.5.

Example 4 A still further batch of sodium alginate was taken and dissolved in distilled water to form a '7 per cent solution which was divided into five parts. To four of the parts, 0.02 per cent, 0.05

per cent, 0.10 per cent and 0.20 per cent of sodium hyposulphite were added respectively while the remaining part was kept as a blank control. The ball-falls of the solutions obtained were as follows:

ball-fall in seconds 1. Blank (i. e. no sodium hyposulphite) 151 2. 0.02 per cent sodium hyposulphite 80.5 3. 0.05 per cent sodium hyposulphite 59.5 4. 0.10 per cent sodium hyposulphite 50 5. 0.20 per cent sodium hyposulphite 43 This example was repeated using softened water instead of distilled water. The results obtained in this case were as follows:

ball-fall in seconds 1. Blank (i. e. no sodium hyposulphite) 92 2. 0.02 per cent sodium hyposulphite 71 3. 0.05 per cent sodium hyphsulphite 5'1 4. 0.10 per cent sodium hyposulphite 39.5 5. 0.20 per cent sodium hyposulphite 35.5

Example 5 For this example, a high viscosity type of sodium alginate was used, a normal '7 per cent aqueous solution of which had a ball-fall of approximately 700 seconds.

100 parts of water, 0.07 part of caustic soda and 0.30 part of sodium hyposulphite were mixed to form a solution at centrigrade, and allowed to stand for 5 minutes. The sodium alginate was then dissolved in the solution in an amount sufficient to form a '7 per cent solution by stirring the alginate and the alkaline sodium hyposulphite solution at 25 centigrade for minutes. The solution obtained had a ball-fall of 66 seconds, that is about one-tenth of a similar solution containing no sodium hyposulphite, and was satisfactorily spun into threads.

In this example, the 5 minute interval imposed between the steps of forming the solution of the sodium hyposulphite and. dissolving the alginate has some effect in reducing the ball-fall since with a 1 minute interval the ball-fall of the solution obtained was 93 seconds and with a 2 minute interval the ball-fall was 89 seconds.

What we claim is:

1. An aqueous alkali-metal alginate spinning solution which contains from about 5 to about 15 per cent by weight of alginate and which also contains, as a viscosity-reducing agent, a salt selected from the group consisting of sodium hyposulphite, potassium hyposulphite and ammonium hyposulphite, the pH value of the said solution being within the range of 6.0 to 9.5.

2. An aqueous alkali-metal alginate spinning solution as claimed in claim 1 wherein the ballfall of the solution, measured as the time in seconds taken for a steel ball of diameter to fall through 20 centimetres of a column of the solution in a tube of 2 centimetres diameter, the temperature of the solution being 25 C., is within the range of about 35 to about '70 seconds.

3. An aqueous alginate spinning solution as claimed in claim 1 wherein the viscosity reducing agent is sodium hyposulphite.

4. An aqueous alkali-metal alginate spinning solution which contains from about 5 to about 15% by weight of alginate and which also contains as a viscosity-reducing agent, a salt selected from the group consisting of sodium hyposulphite, potassium hyposulphite and ammonium hyposulphite.

5. An aqueous alkali-metal alginate spinning solution which contains as a viscosity-reducing agent, a salt selected from the group consisting of sodium hyposulphite, potassium hyposulphite and ammonium hyposulphite.

ERNEST EDWARD TALLIS. HAROLD REGINALD SEPTIMUS CLO'IWORTI-IY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,395,016 Rowland Oct. 25, 1921 2,158,485 Preble May 16, 1939 FOREIGN PATENTS Number Country Date 555,940 Great Britain Sept. 14, 1943 

1. AN AQUEOUS ALKALI-METAL ALGINATE SPINNING SOLUTION WHICH CONTAINS FROM ABOUT 5 TO ABOUT 15 PER CENT BY WEIGHT OF ALGINATE AND WHICH ALSO CONTAINS, AS A VISCOSITY-REDUCING AGENT, A SALT SELECTED FROM THE GROUP CONSISTING OF SODIUM HYPOSULPHITE, POTASSIUM HYPOSULPHITE AND AMMONIUM HYPOSULPHITE, THE PH VALUE OF THE SAID SOLUTION BEING WITHIN THE RANGE OF 6.0 TO 9.5. 